Electrostatic actuator control

ABSTRACT

In one embodiment, a device is provided that includes: a cascaded electrostatic actuator defining a stack in a substrate having a plurality of gaps between parallel plate electrodes; and a controller configured to drive the cascaded electrostatic actuator to open and close selected ones of the gaps.

TECHNICAL FIELD

One or more embodiments relate generally to control ofmicroelectromechanical systems (MEMS) and, more particularly, to controlof MEMS electrostatic actuators.

BACKGROUND

There are two main types or categories of electrostatic MEMS actuators.A first type is denoted as gap-closing or parallel-plate MEMS actuatorswhereas a second type is denoted as an electrostatic comb actuator. Assuggested by the name, a parallel-plate actuator includes two or moreopposing plates. The plates are configured in the actuator such that agap between them is closed as one plate is charged positively (ornegatively) with respect to the opposing plate. Gap-closing actuatorsoffer considerable actuation force as the electrostatic attractionbetween two opposite charges is inversely proportional to the square ofthe separation distance according to Coulomb's law. Thus as the gapseparation is reduced towards zero, the electrostatic attractive forceis markedly increased. Conversely, the electrostatic attractive force ismarkedly lowered as the gap separation is increased from zero. Thus,there is a relatively small range of travel for a conventionalgap-closing actuator as the plates cannot be pulled too far apart fromeach other prior to actuation.

In contrast to gap-closing actuators, the separation between fingers inan electrostatic comb does not change. Rather than have the gap change,an electrostatic comb actuator varies the amount of overlap orinterdigitation between the comb fingers. This interdigitation can occurover a relatively large range, depending upon the length of the combfingers. Thus, comb actuators typically offer much better travel thangap-closing actuators. However, since the gap does not close,electrostatic combs are relatively weak in comparison to gap-closingactuators.

There is thus a need in the art for gap-closing actuators that providethe travel advantages of a comb actuator. In addition, there is a needin the art for the control of such improved gap-closing actuators.

SUMMARY

Methods and systems for controlling microelectromechanical systems(MEMS) actuators, such as cascaded electrostatic actuators, arediscussed. Such electrostatic actuators can be used to move lenses or toactuate shutters in cameras, for example. Such electrostatic actuatorscan be used in any desired application.

In accordance with a first embodiment, a device is provided thatincludes: a cascaded electrostatic actuator defining a stack in asubstrate having a plurality of gaps between parallel plate electrodes;and a controller configured to drive the cascaded electrostatic actuatorto open and close selected ones of the gaps.

In accordance with a second embodiment, a method of controlling acascaded actuator is provided that includes: storing a plurality offirst values corresponding to a plurality of gaps formed by a stack ofparallel plates within a substrate; and driving a selected first one ofthe gaps with a voltage equaling the first value corresponding to thefirst one of the gaps to close the first one of the gaps.

In accordance with a third embodiment, a method of controlling acascaded actuator having a plurality of gaps formed by a stack ofparallel plates within a substrate is provided that includes: drivingthe cascaded actuator with successive pulses of a first voltage tosuccessively close the gaps.

The scope of the invention is defined by the claims, which areincorporated into this Summary by reference. A more completeunderstanding of embodiments of the invention will be afforded to thoseskilled in the art, as well as a realization of additional advantagesthereof, by a consideration of the following detailed description of oneor more embodiments. Reference will be made to the appended sheets ofdrawings that will first be described briefly.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cellular telephone, in accordance with an embodiment.

FIG. 2 is a block diagram of an actuator system, in accordance with anembodiment.

FIG. 3 shows a cascaded electrostatic actuator, in accordance with anembodiment.

FIG. 4 shows a cascaded electrostatic actuator in an unactuated state(with a voltage off), in accordance with an embodiment.

FIG. 5 shows a cascaded electrostatic actuator in an actuated state(with a voltage on), in accordance with an embodiment.

FIG. 6 shows a cascaded electrostatic actuator having two polysiliconhinges, in accordance with an embodiment.

FIG. 7 shows a cascaded electrostatic actuator having one polysiliconhinge, in accordance with an embodiment.

FIG. 8 shows a stack of a cascaded electrostatic actuator, in accordancewith an embodiment.

FIG. 9 shows an enlarged portion of the stack of FIG. 8, in accordancewith an embodiment.

FIG. 10 shows a portion of the stack of FIG. 8, in accordance with anembodiment.

FIG. 11 shows an enlarged portion of the stack of FIG. 10, in accordancewith an embodiment.

FIG. 12 shows a cascaded electrostatic actuator, in accordance with anembodiment.

FIG. 13 shows a cascaded electrostatic actuator, in accordance with anembodiment.

FIG. 14 shows a single cell of a cascaded electrostatic actuator in anunactuated state (with a voltage off), in accordance with an embodiment.

FIG. 15 shows a single cell of a cascaded electrostatic actuator in anactuated state (with a voltage on), in accordance with an embodiment.

FIG. 16 shows a plurality of staggered cells of a cascaded electrostaticactuator in an unactuated state (with a voltage off), in accordance withan embodiment.

FIG. 17 shows a plurality of staggered cells of a cascaded electrostaticactuator in an actuated state (with a voltage on), in accordance with anembodiment.

FIG. 18 shows a position versus voltage diagram for a first cascadedelectrostatic actuator control method.

FIG. 19 shows a position versus voltage diagram for a second cascadedelectrostatic actuator control method.

FIG. 20 a shows a voltage pulse waveform for controlling a cascadedelectrostatic actuator.

FIG. 20 b shows an actuator position responsive to the pulse waveform ofFIG. 20 a.

Embodiments of the invention and their advantages are best understood byreferring to the detailed description that follows. It should beappreciated that like reference numerals are used to identify likeelements illustrated in one or more of the figures.

DETAILED DESCRIPTION

To provide greater travel in a gap-closing actuator design, gap-closingactuators are stacked to provide what is denoted herein as a cascadedelectrostatic actuator. Systems and methods are disclosed herein toprovide control of microelectromechanical systems (MEMS) cascadedelectrostatic actuators and applications therefor. In accordance with anembodiment of the invention, a cascaded electrostatic actuator cancomprise a plurality of alternating fingers, electrodes, plates, orlayers upon which opposite charges can be placed so as to cause thealternating layers to move toward one another.

Embodiments of the actuator can be controlled, for example, to move ashutter in a miniature camera to at least partially define an exposureand/or to move one or more lenses in a miniature camera to effect focus,zoom, or optical image stabilization (OIS). Embodiments of the actuatorcan be used to move or actuate various MEMS and non-MEMS devices.

FIG. 1 shows a cellular telephone 100, in accordance with an embodiment.The cellular telephone 100 can comprise a miniature camera 101. Theminiature camera 101 can comprise a cascaded electrostatic actuator 102controlled to move at least one lens (such as lens 2801 of FIG. 28) of alens assembly 103. Such movement can effect focusing, zooming, and/orimage stabilization, for example.

FIG. 2 is a block diagram of a cascaded electrostatic actuator system,in accordance with an embodiment. The cascaded electrostatic actuator102 can move a load 201. The load 201 can be the lens 2801 of the lensassembly 103 for FIG. 1, for example. The actuator 102 is controlled bya controller 203.

The controller 203 can be a microprocessor, such as a custommicroprocessor or a general purpose microprocessor. The controller 203can be dedicated to the operation of the actuator 102 or the controller103 can also provide other functionality, such as functionally commonlyassociated with at least some portion of the operation of the cellulartelephone 100 of FIG. 1.

A sensor 202 can sense the position, velocity, acceleration, and/or anyother desired parameter associated with the load 201. The sensor 202 cansense the position of the lens 2801 (see FIG. 28) of lens assembly 103so as to facilitate focusing of the camera 101, for example.

The sensor 202 can provide an output representative of the sensedparameter to the controller 203. The controller 203 can use the outputof the sensor 202 to facilitate focusing of the camera 101, for example,as discussed further herein.

The controller 203 can control a display 204. The display 204 caninclude any desired information. For example, the display 204 can showthe scene being photographed, can indicate whether an autofocus functionis on or off and/or can indicate what portion of a scene beingphotographed is being used as the target for autofocusing.

User controls 206 can affect operation of the actuator 102, via thecontroller 203. For example, a user can operate the user controls 206 tozoom, turn autofocus on or off, and/or turn image stabilization on oroff.

A memory 205 can store programs for the controller 203 and/or can storeother information. For example, the memory 205 can store images capturedby the camera 101, parameters related to autofocus such as distance tothe subject, and/or parameters for relating values sensed by the sensor202 to positions of the lens 2801 (see FIG. 28).

The cascaded electrostatic actuator control techniques disclosed hereinmay be applied to any stacked parallel-plate actuator. However, aparticularly advantageous stacked architecture is disclosed inconcurrently-filed U.S. application Ser. No. 13/247,847, entitled“Cascaded Electrostatic Actuator,” the contents of which areincorporated by reference. In this cascaded architecture, the parallelplates are defined in a semiconductor substrate. For example, a seriesof parallel plates may be defined in a substrate with regard to aserpentine first electrode and a second electrode. Fingers in the secondelectrode interdigitate with folds in the serpentine first electrode todefine the parallel plates in the plane of the semiconductor substrate.A first pair of parallel plates may defined by a first fold in theserpentine first electrode and a first finger for the second electrode.A second pair of parallel plates is defined by a second fold in theserpentine first electrode and a second finger for the second electrode,and so on. In this fashion, any number of parallel plate pairs may bedefined by corresponding folds in the serpentine first electrode andfingers in the second electrode.

Numerous other cascaded architectures may be formed in the plane of thesemiconductor substrate. For example a series of isolated plates may beformed through suitable etching of the substrate. Each isolated plate issurrounded by a circumferential gap that separates the plate from thesurrounding substrate. The isolated plates and surrounding gaps formcells that may be stacked so that plates of substrate separate thecells. In this fashion, each isolated plate within the stack faces twoopposing plates of substrate. Cascaded electrostatic actuatorarchitectures are advantageous in that they are readily constructedusing MEMS semiconductor manufacturing techniques as discussed furtherin the incorporated Cascaded Electrostatic Actuator application.

An embodiment of the serpentine cascaded electrostatic actuator is shownin FIG. 3. A serpentine cascaded electrostatic actuator 103 includes aserpentine first electrode 301. Each serpentine fold in electrode 301corresponds to a pair of parallel plates in a series of such pairs. Eachpair of parallel plates is formed by a fold in serpentine electrode 301and a finger from a second electrode 302. For example, a first pair ofparallel plates is defined by a first finger plate 315 for electrode 302and a first fold plate 316 for electrode 302. Similarly, a second pairof parallel plates for actuator 103 corresponds to a second fold plate326 and a second finger plate 320. A third pair of parallel platescorresponds to a third finger plate 323 and a third fold plate 323, andso on. In this fashion, each serpentine fold in electrode 301corresponds to a pair of parallel plates in actuator 103. Electrode 302is separated from electrode 301 by a gap 303.

To provide ease of manufacture, a single crystalline silicon substratemay be etched using conventional MEMS techniques to form what willbecome electrode 301. The resulting trench surrounding electrode 301 maybe partially filled with polysilicon to form electrode 302 as discussedfurther herein. Note that electrode 301 may be polysilicon in that aserpentine trench may be etched that then partially filled withpolysilicon. The surrounding silicon portion of the wafer will thus formfingers of electrode 302 that cooperate with folds in electrode 301 toform pairs of parallel plates.

Electrode 302 forms opposing fingers that interdigitate with theserpentine folds of electrode 301. For example, finger 320 for electrode302 opposes finger 315. If there is an integer number N of serpentinefolds to define a stack 304, then there are N gaps 303 that may becollapsed during actuation. Stack 304 has an un-actuated (no voltageapplied) height, Dimension L.

Each gap 303 can have a un-actuated width, Dimension G. With Nserpentine fold in stack 304, there is thus a potential travel of N*Gdefining the range of motion for actuator 103. In general, the travel ofthe actuator 102 can be approximately the width, Dimension G, of the gap303 for a rest or un-actuated state for actuator 102 multiplied by thenumber of gaps 303.

A base 305 at one end of the stack 304 can define a proximal end 321 ofthe actuator 102. The opposite end of the stack 304 can define a distalend 322 of the actuator 102. The base 305 can be attached to onestructure and the distal end 322 of the stack 304 can be attached toanother structure such that actuation of the actuator effects relativemovement of the two structures.

For example, the base 305 can be attached to a stationary portion (suchas the lens barrel 2803 of FIG. 28) of the lens assembly 103 and thedistal end 322 of the stack 304 can be attached to the lens 2801 (seeFIG. 28) to effect movement of the lens 2801 for the focusing of thecamera 101. The base 305 can also facilitate electrical connection tothe actuator 102, as discussed herein.

Electrical contract to the actuator 102 can be made in any desiredmanner. For example, electrical contact can be made to a pad 311 formedof single crystalline silicon which can be in electrical communicationwith first electrode 301 and electrical contact can be made to thesurrounding structure 312 formed of polysilicon which can be inelectrical communication with the second electrode 302.

Actuation of the actuator 102 can result in sequential or simultaneousclosure of the various gaps 303 due to the electrostatic force betweenthe electrodes 301 and 302. By varying the length of the fingers and/orthe width of the fingers and corresponding serpentine folds, the ease orresistance for which any given gap 303 will close in response toelectrostatic attraction between electrodes 301 and 302 may be varied asdesired. As used herein, a gap closure is referred to as a snap-inmotion. During the snap-in motion, the distal end of the stack 304 movesquickly or snaps from its distal most or unactuated position to itsproximal most or actuated position. In one embodiment, during thesnap-in motion, substantially all the N gaps 303 corresponding to stack304 move substantially simultaneously toward one another such that thestack 304 rapidly contracts in length, Dimension L.

When unactuated, the stack 304 is expanded (has approximately thelongest length, Dimension L, thereof). When actuated, the stack 304 iscontracted (has approximately the shortest length, Dimension L,thereof).

Rather than all or nothing snap-in movement of the stack 304 as a whole,separate snap-in movement of separate portions or segments of the stack304 can be provided. In this manner, more gradual, controlled movementof the actuator 102 can be provided. Incremental or partial actuation ofthe actuator 102 can be provided in this manner. Generally continuousactuation of the actuator 102 can be provided in this manner.

For example, the different segments of the stack 304 can have differentstiffness, such that the different segments of the stack 304 snap-in atdifferent voltages. In general, the voltage necessary for any given gap303 to close is denoted as the pull-in voltage for that gap 303 andcorresponding parallel plate pair. Thus, as the voltage is increased,different segments of the stack 304 snap-in and the length of the stack304 changes more gradually.

Different stiffnesses of the different segments of the stack 304 can beprovided by fabricating the electrode 301 and/or the electrode 302 so asto have different widths within the different segments. Differentstiffnesses of the different segments of the stack 304 can be providedby fabricating the electrode 301 and/or the electrode 302 so as to havedifferent shapes within the different segments.

Different widths, Dimension G, of the gap 303 can be used to providedifferent forces between given fingers of electrode 302 andcorresponding surrounding serpentine folds of electrode 301 such thatdifferent gaps 303 snap in at different times (upon the application ofdifferent voltages). In this manner, smoother operation of the actuator102 can be provided.

The motion of the actuator 102 need not be snap-in, either as a whole orfor segments thereof. For example, the stiffness of the stack 304 can besubstantially continuously non-linear such that the motion of theactuator 102 is substantially continuous. Thus, the distal end of thestack 304 can move generally continuously as the voltage applied to theactuator 102 is increased.

The position or state of the actuator 102 (such as the position of thedistal end 322 of the stack 304) can be determined by measuring thecapacitance of the actuator 102. That is, the capacitance betweenelectrode 301 and electrode 302 can provide an indication of whether ornot the actuator 102 is actuated and can provide an indication of thedegree of actuation. The position of an portion of the actuator 102 canbe determined by measuring the capacitance of that portion.

The actuator 102 can be fabricated by etching a trench in a singlecrystalline substrate. The un-etched portions of the substrate candefine electrode 301. Alternatively, the trench can define electrode301. Thus, the trench can be filled with polysilicon to define electrode302 or electrode 301, depending upon which element the trench defines.An oxide layer can be formed to electrically isolate the electrodesduring operation of the actuator 102. The fabrication process isdescribed in further detail herein.

Operation of the cascaded electrostatic actuator 102 is discussed withreference to FIGS. 4 and 5. FIG. 4 shows the actuator 102 in anunactuated state (with a voltage off), in accordance with an embodiment.FIG. 5 shows the actuator 102 in an actuated state (with a voltage on),in accordance with an embodiment.

With particular reference to FIG. 4, when no voltage is applied acrosselectrodes 301 and 302, then the charges on the electrodes areapproximately the same. That is, electrodes 301 and 302 are atapproximately the same electrical potential. Thus, there is nosubstantial force exerted between them. Since there is no substantialforce exerted, the actuator 102 remains in an unactuated state.

With particular reference to FIG. 5, when a voltage is applied acrosselectrodes 301 and 302, then the charges on the electrodes aresubstantially different with respect to one another. That is, electrodes301 and 302 are at substantially different electrical potentials. Thus,there is a substantial attractive force exerted between electrodes 301and 302. Since there is a substantial force exerted, the actuator 102moves or snaps in to an actuated state. In the actuated state, the stack304 is compressed or contracted with respect to the unactuated state.

When the stack 304 is contracted, gaps 303 close. An insulator, such asan oxide layer 421 can be formed upon one or both of the electrodes 301and 302 to inhibit shorting or electrical contact therebetween.

The height, Dimension A, of the stack 304 of the unactuated actuator 102of FIG. 4 is substantially greater than the height, Dimension B, of thestack 304 of the actuated actuator 102 of FIG. 5. Thus, duringactuation, the distal end 322 of the stack 304 moves toward the base 305of the actuator 102.

The amount of such movement can be approximately equal to the sum of thegaps 303 of the stack 304. Thus, the amount of such movement can begreater than the width of a single gap 303. An advantage of thiscascaded configuration of the actuator 102 can be that more travel canbe obtained by making the stack 304 thereof longer. That is, as moregaps 303 are added to stack 304, the total amount of travel of thedistal end 322 of the stack 304 obtained when the actuator 102 isactuated is proportionally increased.

Another advantage of this cascaded configuration of the actuator 102 canbe that more force can be provided. The electrostatic force provided bysuch an actuator is proportional to the cross-sectional area of theelectrodes 301 and 302. Thus, increased force can be obtained by makingthe thickness, Dimension T of FIG. 8 and/or the width, Dimension W ofFIG. 8, greater. Substantial forces can be provided by the cascadedconfiguration. For example, a 1 mm wide, 150 μm thick actuator 102 canproduce approximately 10 grams of force.

A first electrical contact 401 and a second electrical contact 402 canbe formed from or upon the base 305. For example, the first electricalcontact 401 can be formed from the material of the first electrode 301,e.g., single crystalline silicon, and the second electrical contact 402can be formed from the material of the electrode 302, e.g., polysilicon.

The first electrical contact 401 can be formed along with the firstelectrode 301 during the fabrication process and can thus be inelectrical contact with electrode 301. The second electrical contact 402can be formed upon the base 305 after the first electrode and the secondelectrode have been fabricated, as discussed herein. The secondelectrical contact 402 can be electrically insulated from the base 305,such as via an oxide layer (not shown) formed therebetween. The secondelectrical contact 402 can be in electrical contact with the secondelectrode 302, such as where the second electrical contact 402 is formedthereover.

Electrical connection can be made to the first electrical contact 401and the second electrical contact 402 in the manner that electricalconnection is commonly made to the pads or electrical contacts ofintegrated circuits. For example, such electrical connection can be madevia wire bonding.

FIG. 6 shows a cascaded electrostatic actuator 102 having a firstpolysilicon hinge 601 and a second polysilicon hinge 602, in accordancewith an embodiment. The first polysilicon hinge 601 and the secondpolysilicon hinge 602 cooperate to cause the distal end 322 of the stack304 to flex or bend downwardly, out of the plane of the actuator 102during actuation. In this manner, a more complex, non-linear motion canbe obtained.

The motion of the distal end of the stack 304 can depend, at leastpartially, upon the stiffness of the first polysilicon hinge 601 and thesecond polysilicon hinge 602. The stiffness of the first polysiliconhinge 601 and the second polysilicon hinge 602 can depend upon the widthand thickness (e.g., the cross-sectional area) thereof.

The motion of the distal end of the stack 304 can have bothtranslational (linear) and rotational (non-linear) components.Generally, the stiffer the first polysilicon hinge 601 and the secondpolysilicon hinge 602, the less the translational component will be andthe greater the rotational component will be. The first polysiliconhinge 601 and the second polysilicon hinge 602 can have differentstiffnesses, such that more complex motion of the distal end of thestack 304 can be provided.

The first polysilicon hinge 601 and the second polysilicon hinge 602inhibit or prevent the upper surface 603 of the stack 304 fromcontracting when the actuator 102 is actuated. As the lower surface ofthe actuator contracts, generally along the centerline or movement axis611, the distal end of the stack 304 curls down along an arc,approximately about an axis 612.

Thus, the distal end of the stack 304 can have a rotational component,and can have some linear component as well (depending upon the stiffnessof the first polysilicon hinge 601 and the second polysilicon hinge602). Such motion can be desirable in those instances where linearmotion is inadequate. The use of an actuator that directly providesmotion with such a rotational component has the advantage of notrequiring additional structure to convert the motion from a linearactuator into a desired non-linear motion.

FIG. 7 shows a cascaded electrostatic actuator having one polysiliconhinge, i.e., the first 601 polysilicon hinge, in accordance with anembodiment. The first polysilicon hinge 601 can cause the distal end ofthe stack 304 to flex or bend downwardly, out of the plane of theactuator 102, and to twist at the same time. Such twisting can result insome lateral movement of the distal end of the stack 304. In thismanner, a more complex, non-linear motion can be obtained.

The motion of the distal end of the stack 304 can depend, at leastpartially, upon the stiffness of the first polysilicon hinge 601. Thestiffness of the first polysilicon hinge 601 can depend upon the widthand thickness (e.g., the cross-sectional area) thereof.

The motion of the distal end of the stack 304 can have bothtranslational (linear) and rotational (non-linear) components. Therotational components can be about two or more separate axes. Generally,the stiffer the first polysilicon hinge 601, the less the translationalcomponent will be and the greater the rotational components will be.

The use of only the first polysilicon hinge 601 adds stiffness to thestack 304 asymmetrically. Such asymmetric stiffness result in the morecomplex bending and twisting motion of the stack 304 during actuationand de-actuation.

The first polysilicon hinge 601 can inhibit or prevent the one side (theleft side as shown in FIG. 7) of the upper surface 603 of the stack 304from contracting when the actuator 102 is actuated. As the lower surfaceand right side of the actuator contract generally along the centerlineor movement axis 611, the distal end of the stack 304 curls down alongan arc, such as about axis 612, and also twists, such as about acenterline or axis 611.

Thus, the distal end 322 of the stack 304 can have two rotationalcomponents (bending and twisting), and can have some linear component aswell (depending upon the stiffness of the first polysilicon hinge 601and/or the second polysilicon hinge 602. Such motion can be desirable inthose instances where linear motion is inadequate. The use of anactuator that directly provides motion with such a rotational componenthas the advantage of not requiring additional structure to convert themotion from a linear actuator into a desired non-linear motion.

FIG. 8 shows the stack 304 (or a portion of the stack 304) inperspective, in accordance with an embodiment. The stack 304 can have athickness, Dimension T; a width, Dimension W; and a length, Dimension L.The stack 304 is made up of the first electrode 301 and the secondelectrode 302. Consequently, each of the electrodes have approximatelythe same thickness, Dimension T and approximately the same width,Dimension W.

With respect to each gap 303, electrodes 301 and 302 have opposing facesof thickness T and width W. The area of these opposing surfaces, inpart, determines the electrostatic force generated across gap 303.Generally, the greater this area is, the greater the electrostatic forceis across each gap 303.

FIG. 9 shows an enlarged portion of the stack 304 of FIG. 8, inaccordance with an embodiment. The serpentine fold plates for electrode301 can have a thickness, Dimension D, which can be approximately 6 μm,for example. The finger plates formed by electrode 302 can have athickness, Dimension E, which can be approximately 6 μm, for example.The thickness, Dimension D, of the serpentine structure for the firstelectrode can be the same as the thickness, Dimension E, of the fingersfor the second electrode. Alternatively, these thicknesses may differ.

The gap 303 can have a thickness, Dimension G, which can beapproximately 1 μm, for example. The widths of the gaps 303, DimensionG, in part, determine the electrostatic force generated across the gaps.Generally, the smaller the gap 303 is, the greater the electrostaticforce is between opposing faces for electrodes 301 and 302.

The widths of the gaps, Dimension G, can all be the same. The widths ofthe gaps, Dimension G, can be different with respect to one another.

FIG. 10 shows a cross-section of the stack 304 taken alone line 10 ofFIG. 8, in accordance with an embodiment. The relationship between theenclosing serpentine folds for first electrode 301, the fingers forsecond electrode 302, and the gaps 303 can clearly be seen.

FIG. 11 shows an enlarged portion of the stack 304 of FIG. 10, inaccordance with an embodiment. The gap 303 can be an air gap.Alternatively, the gap 303 can be filled or partially filled with areadily compressible material. For example, the gap 303 can contain asubstantial vacuum or an inert gas.

When a voltage is applied across electrodes 301 and 302, anelectrostatic force is generated therebetween. This electrostatic forceis attractive since voltages of different polarities result inattraction. This attractive force tends to cause the stack 304 tocollapse or contract. As the stack 304 contracts, the width, DimensionG, of the gap 303 is reduced.

The width, Dimension G, of for a given gap 303 can be reduced tosubstantially zero, at which point the adjacent faces for electrodes 301and 302 can contact one another. As discussed herein, a oxide layer canbe formed upon the first layers 301 and/or the second layers 302 toprevent shorting of the charges thereon.

According to an embodiment, as different, e.g., higher, voltages areapplied to the actuator 102, different gaps 303 can close at differenttimes. Such serial contracting of the stack 304 can provide a morecontrolled use of the actuator 102, as discussed herein.

Rather than use a serpentine architecture, a cascaded electrostaticactuator may be formed using isolated plates as discussed above. In suchan isolated plate architecture, a plurality of isolated plates form thecore of a series of cells. Each cell includes an isolated plateelectrode surrounded or partially surrounded by a circumferential gap.Successive isolated plates are separated by plates of substrate suchthat a given isolated plate has a first face facing a first substrateplate and a second face facing a second substrate plate. For example,FIG. 12 shows a cascaded electrostatic actuator 1200 using such isolatedplates, in accordance with an embodiment. According to this embodiment,a plurality of isolated plate electrodes 1201 are circumferentiallysurrounded by a corresponding substrate plate electrodes 1202. Theisolated plates 1201 are surrounded by circumferential gaps 1207.

Since plates 1201 are circumferentially surrounded by gap 1202, they aresupported by at their ends by surface polysilicon flexures, such as afirst surface flexure 1203 and a second surface flexure 1204. The firstsurface flexure 1203 and the second surface flexure 1204 can secureplates 1201 from plate electrodes 1202 so as to inhibit relative motiontherebetween in the vicinity of the surface flexures.

Each isolated plate electrode and corresponding substrate plateelectrodes form a cell (shown as element 1400 in FIG. 14). Cells 1400are stacked to form a stack 1205. Flexures 1203 and 1204 secure the endsof each isolated plate within a cell with respect to the correspondingloop electrode. The flexures prevent closure of gap 1207 in the vicinityof the flexures.

In this manner, less-contracting or non-contracting portions of theelectrostatic actuator 1200 can be defined. That is, the first surfaceflexure 1203 and the second surface flexure 1204 can inhibit or preventthe stack 1205 from contracting along edges 1221 and 1222 thereof whileallowing the stack 1205 to contract in a central portion 1223 thereof.

Electrical contract to the actuator 1200 can be made via a pad orelectrical connection 1211 formed of polysilicon that can be inelectrical communication with isolated plates 1201 and which can beelectrically isolated from the substrate and thus from substrate plateelectrodes 1202. In this fashion, each cell may be selectively activatedor groups of cells may be selectively activated. Alternatively, theentire ensemble of the cells may all be selectively activated.

Electrical contact to substrate plate electrodes 1202 may be made via asubstrate or electrical connection 1212 formed of single crystallinesilicon, which can be electrically isolated from the isolate plateelectrodes 1201. Substrate plate electrodes 1202 and isolated plateelectrodes 1201 form pairs of opposing plates across each gap 1207.

FIG. 13 shows a cascaded electrostatic actuator 1300, in accordance withan embodiment. As discussed analogously with regard to FIG. 12, isolatedplate electrodes 1301 alternate with substrate plate electrodes 1302. Asseen in FIG. 14, a cell 1400 is thus formed by as single isolated plateelectrode 1301 and a surrounding substrate plate electrodes 1302. Theisolated plates 1301 are separated from the surrounding substrate plateelectrodes by gaps 1307.

The isolated plate electrodes 1301 and the substrate plate electrodes1302 can be supported by surface polysilicon flexures, such as a firstsurface flexure 1303 and a second surface flexure 1304. The firstsurface flexure 1303 and the second surface flexure 1304 can secure theisolated plate electrodes 1301 and the substrate plate electrodes 1302to one another so as to inhibit relative motion therebetween.

In this manner, non-contractable portions of a stack 1305 can bedefined. That is, the first surface flexure 1303 and the second surfaceflexure 1304 can inhibit or prevent the stack 1305 from contractingalong edges 1321 and 1322 thereof while allowing the stack 1305 tocollapse in a central portion 1323 thereof.

Electrical contract to the actuator 1300 can be made via the firstsurface flexure 1303 formed of polysilicon, which can be in electricalcommunication with the isolated plates 1301 and which can beelectrically isolated from the substrate plates 1302.

Electrical contact 1312 may be formed of single crystalline silicon,which can be in electrical communication with the substrate plates 1302and which can be electrically isolated from the isolated plates 1301.

FIGS. 14 and 15 show a single cell 1400, such as from the cascadedelectrostatic actuators for FIGS. 12, 13, 16, and 17. Each of the cells1400 of the actuators of FIGS. 12, 13, 16, and 17, for example, can havean un-actuated configuration and an actuated configuration, as discussedbelow.

Each cell can be approximately 200 μm long by approximately 20 μm wide,for example. Each cell can have any desired dimensions.

Each cell can be generally oval or elongated in shape. Each cell canhave any desired shape.

With particular reference to FIG. 14, a single cell 1400 of an actuatoris in an un-actuated state (with a voltage off), in accordance with anembodiment. The single cell 1400 can comprise a flexure (notillustrated), an isolated plate 1301, and a pair of surroundingsubstrate plates 1302.

In the un-actuated state, the circumferential gap 1307 can have asubstantially uniform width. With particular reference to FIG. 15 asingle cell 1400 of a cascaded electrostatic actuator is in an actuatedstate (with a voltage on), in accordance with an embodiment. In theactuated state, the cell 1400 has contracted such that central portions1405 of the substrate plates 1302 are closer to, e.g. touching or almosttouching, the isolated plate 1301. The gap 1307 is still defined,however, at end portions 1403. The isolated plate 1301 and/or thesurrounding substrate plates 1302 can have an insulator, e.g., an oxidelayer (not shown) formed thereon to electrically insulate the isolatedplate 1301 from the surrounding plates 1302 and thus prevent shorting ofthe electrostatic actuator.

FIGS. 16 and 17 show a plurality of cells 1400 that are fabricatedtogether so as to define a stack 1600 for a cascaded electrostaticactuator. Any desired number of cells 1400 in any desired configurationcan be used to define the stack 1600. The stack 1600 can have anydesired number of cells 1400 in a row and can have any desired width.The stack 1600 can have any desired number of cells 1400 in a column andcan have any desired height.

FIG. 16 shows a plurality of cells 1400 of a stack 1600 of a cascadedelectrostatic actuator in an unactuated state (with a voltage off), inaccordance with an embodiment. The isolated plates 1301 and thecorresponding substrate plates 1302 of each of the cells 1400 aresubstantially straight and parallel with respect to one another.Alternatively, the plates 1301 and 1302 can be crooked and/ornon-parallel with respect to one another.

The cells 1400 of the stack 1600 can be somewhat analogous to the cellsof a muscle. Providing more cells can provide more travel and/or moreforce. Generally, providing more cells 1400 in each column will providemore travel and providing more cells 1400 in each row will provide moreforce.

The cells 1400, as shown in FIG. 16, are in a staggered configuration.That is, adjacent columns of cells 1400 overlap substantially withrespect to one another. Alternatively, the cells 1400 can have anon-staggered configuration.

The cells 1400, as shown in FIG. 16, are staggered so as to haveapproximately 50% overlap. The cells 1400 can be staggered in adifferent fashion, so as to have any desired amount of overlap. Forexample, the cells 1400 can have 20% overlap, 25% overlap, 33.3%overlap, or any other amount of overlap.

The stack 1600 is at approximately its full height, e.g., isapproximately fully extended. Since the voltage is off and no charge isbeing applied to plates 1301 and 1302, the stack has not contracted.

FIG. 17 shows a plurality of cells 1400 of a stack 1600 of a cascadedelectrostatic actuator in an actuated state (with a voltage on), inaccordance with an embodiment. The surrounding plates 1302 for each ofthe cells 1400 is substantially curved inwardly toward the isolatedplate 1301 (as shown also in FIG. 15). Each of the cells 1400 is fullycontracted. The stack 1600 is at approximately its shortest height,e.g., is approximately fully contracted. Note that as compared to theserpentine embodiments discussed earlier, stack 1600 has just one-halfthe travel since the overlapped ends of each cell cannot contract.However, cells 1400 may be made electrically independent of each otherby having paths of polysilicon of other conductive leads patterned todefine conductors to just certain ones or groups of cells. Thus, anisolated plate embodiment offers attractive selective actuationcapabilities as compared to a serpentine embodiment.

Although FIG. 17 shows all of the cells 1400 in an actuated or fullycontracted state, some of the cells 1400 can alternatively remain in anunactuated state due to the selective capability of an isolated plateembodiment. In this manner, the height of the stack 1600 can be moreprecisely controlled. For example, every other row of the cells 1400 canbe actuated to provide approximately one half of the total travel of theactuator.

Further, the stack can be made to curve by actuating some cells 1400while not actuating other cells 1400. For example, the cells 1400 on theleft side of the stack 1600 can be actuated while the cells 1400 on theright side of the stack 1600 are not actuated to cause the stack 1600 tobend to the left. Additional details regarding cascaded actuators aredisclosed in the incorporated-by-reference Cascaded ElectrostaticActuator U.S. patent application.

The novel properties of the cascaded gap-closing actuators discussedabove require novel control techniques. For example, referring again toFIG. 2, controller 203 may be configured to command an application of asufficiently high voltage such as 30 V across the various gaps in thecascaded electrostatic stack to snap-in all the gaps and collapsecascaded electrostatic actuator 102 to it fully actuated state.Conversely, should controller 203 apply a voltage of 0 V across allgaps, actuator 102 may relax to its un-actuated height. Actuator 102 insuch an embodiment would be essentially a two-position actuator.

However, for applications such as autofocus it is desirable to achievemore granularity in the achievable actuation positions with a highdegree of confidence. For example, in a serpentine embodiment, theserpentine folds may be of varying length such that the stack forms atrapezoidal shape. An analogous length variation may be implemented inan isolated plate cascaded electrostatic actuator embodiment. Regardlessof the actuator type, the varying lengths of the resulting plate pairsthus have varying resistances to snap-in. Once a given gap has closed,the resulting greatly increased electrostatic attraction will maintainthe gap closed at voltages lower than that required for the initialsnap-in. There is thus hysteresis with respect to the voltages forsnap-in versus the voltage at which closed gaps open.

The resulting serpentine cascaded actuator may thus be selectivelyactuated across various intermediate positions between closure of allgaps and having all gaps completely relaxed. Each gap will snap-in atdiffering voltages. The more flexible plates will snap-in at lowervoltages whereas the stiffer plates will snap-in at higher voltages. Thegaps will then open at correspondingly lower voltages according to theactuator's hysteresis. The resulting actuator could then be controlledresponsive to this hysteresis. For example, controller 203 may control avariable serpentine cascaded actuator as shown by FIG. 18. The actuatorcan be selectively actuated over a range of intermediate positions tocontract from a relaxed position (O μm) to a fully actuated position(100 μm). The various voltage may be stored in a memory or look-up tablewithin controller 203. For example, an initial gap closes at the initialsnap-in voltage. The actuator travel is thus approximately 5 μm for thatinitial gap closure. As the voltage is increased, additional gaps areclosed such that the full travel of 100 μm is achieved at 30 V. Shouldthe actuator need to expand from any given position, the hysteresis isaccounted for by the look-up table. For example, to relax the initialgap closure, the voltage would be decreased as shown by path A to placethe actuator back into the completely un-actuated state.

Note that the accounting for such hysteresis may be undesirable incertain applications. Thus, the serpentine cascaded electrostaticactuator may be configured to have the travel versus voltage behaviorshown in FIG. 19. After the actuator has been completely actuated, thereis a voltage at which the actuator will relax an initial gap, which isdesignated herein as the initial snap-out voltage. In the embodiment ofFIG. 19, there is a gap between the initial snap-in voltage and theinitial snap-out voltage. In contrast, the voltage control behaviorshown in FIG. 18 has no such gap. Because of the gap, a controller coulduse a voltage within the gap (e.g., 15V) to maintain a given position.So long as the actuator is held at the gap voltage, no gaps will closeor relax—the actuator will maintain whatever current actuation state ithas. The actuator could then be stepped up by another gap closure by apulsed maximum voltage such 30 V. In contrast, a short pulse of 0 Vwould relax the actuator by a selected gap. In this fashion, controller203 controls the cascaded actuator analogously to a linear steppermotor.

As seen in FIG. 20 a, controller 203 commands high voltage pulses (e.g.,30V) to close gaps sequentially. Thus, as seen in FIG. 20 b, a gapcloses corresponding to each high pulse in FIG. 20 a. Conversely,controller 203 commands low voltage pulses (e.g, 0V) to open gapssequentially.

While the invention has been described in detail in connection with onlya limited number of embodiments, it should be readily understood thatthe invention is not limited to such disclosed embodiments. Rather, theinvention can be modified to incorporate any number of variations,alterations, substitutions or equivalent arrangements not heretoforedescribed, but which are commensurate with the spirit and scope of theinvention. Additionally, while various embodiments of the invention havebeen described, it is to be understood that aspects of the invention mayinclude only some of the described embodiments. Accordingly, theinvention is not to be seen as limited by the foregoing description, butis only limited by the scope of the appended claims.

What is claimed is:
 1. A device comprising: a cascaded electrostaticactuator defining a stack in a substrate having a plurality of gapsbetween parallel plate electrodes; and a controller configured to: drivethe cascaded electrostatic actuator to open and close selected ones ofthe gaps, wherein an actuation distance of the cascaded electrostaticactuator is determined based on a number of open gaps and a number ofclosed gaps; and drive the cascaded actuator with pulses of a firstvoltage to close selected ones of the gaps and to drive the cascadedelectrostatic actuator with pulses of a second voltage to open selectedones of the closed gaps.
 2. The device of claim 1, wherein thecontroller is further configured to drive the cascaded actuator tosimultaneously open all the gaps and to simultaneously close all thegaps.
 3. The device of claim 1, wherein the gaps are arranged from afirst gap to a last gap, and wherein the controller is furtherconfigured to drive the cascaded actuator to successively close the gapsbeginning with the first gap and ending with the last gap.
 4. The deviceof claim 1, wherein the gaps are arranged from a first gap to a lastgap, and wherein the controller is configured to drive the cascadedactuator to successively open the gaps beginning with the last gap andending with the first gap, and wherein a position of the cascadedactuator is determined based on a sum of the closed gaps minus a sum ofthe open gaps.
 5. The device of claim 1, further comprising a memorystoring a plurality of first values corresponding to the plurality ofgaps, and wherein the controller is further configured to close selectedones of the gaps by driving the cascaded actuator with a voltageequaling the corresponding first value stored in the memory.
 6. Thedevice of claim 5, wherein the memory stores a plurality of secondvalues corresponding to the plurality of gaps, and wherein thecontroller is further configured to open selected ones of the gaps bydriving the cascaded actuator with a voltage equaling the correspondingsecond value stored in the memory.
 7. The device of claim 1, wherein thecontroller is further configured to drive the cascaded actuator with aconstant third voltage to maintain an open or closed status for each ofthe gaps.
 8. The device of claim 7, wherein the first voltage is greaterthan the second voltage, and wherein the third voltage lies between thefirst and second voltages.
 9. The device of claim 8, wherein firstvoltage is approximately 30 V and the second voltage is approximatelyground.
 10. A method of controlling a cascaded electrostatic actuator,comprising: storing a plurality of first values corresponding to aplurality of gaps formed by a stack of parallel plates within asubstrate; and driving a selected first one of the gaps with a voltageequaling the first value corresponding to the first one of the gaps toclose the first one of the gaps of the cascaded electrostatic actuator,wherein an actuation distance of the cascaded electrostatic actuator isdetermined based on a number of open gaps and a number of closed gaps.11. The method of claim 10, further comprising: storing a plurality ofsecond values corresponding to the plurality of gaps; and driving aselected second one of the gaps with a voltage equaling the second valuecorresponding to the second one of the gaps to close the second one ofthe gaps.
 12. The method of claim 11, wherein a gap range exists betweenthe first values and the second values, the method further comprisingdriving each of the gaps with a voltage equaling a value within the gaprange to maintain an open and closed status for the gaps.
 13. The methodof claim 10, wherein the parallel plates are defined between a foldedserpentine electrode and a second electrode forming interdigitatedfingers with the folds in the folded serpentine electrode.
 14. Themethod of claim 10, wherein the parallel plates are defined between aplurality of isolated plates and substrate surrounding the isolatedplates.
 15. A method of controlling a cascaded electrostatic actuatorhaving a plurality of gaps formed by a stack of parallel plates within asubstrate, the method comprising: driving the cascaded electrostaticactuator with successive pulses of a first voltage to successively closethe gaps, wherein an actuation distance of the cascaded electrostaticactuator is determined based on a number of open gaps and a number ofclosed gaps; and driving the cascaded electrostatic actuator withsuccessive pulses of a second voltage to successively open the gaps. 16.The method of claim 15, wherein the first voltage is greater than thesecond voltage.
 17. The method of claim 16, further comprising: drivingthe cascaded electrostatic actuator with a third voltage to maintain anopen or closed status for the gaps.
 18. The method of claim 17, whereinthe third voltage is less than the second voltage and greater than thefirst voltage.